A variety of products and articles, including, for example, medical instruments, devices, and equipment, must be sterilized prior to use to prevent bio-contamination of a wound site, a sample, an organism, or the like. A number of sterilization processes are used that involve contacting the product or article with a sterilant. Examples of such sterilants include steam, ethylene oxide, hydrogen peroxide, and the like. Steam sterilization is widely used, at least in part because multiple batches of articles can be subjected to sterilization conditions during a 24 hour period using a single steam sterilizer.
Monitoring for conditions sufficient for sterilization is generally carried out by placing an appropriate sterilization indicator along with the product and/or article to be sterilized within a sterilization chamber. A variety of sterilization indicators, including chemical and biological indicators, are known and used for this purpose. Chemical indicators offer an advantage in that they can be read immediately at the end of a sterilization process. Internal chemical indicators are placed within an instrument tray and are read in the operating room upon opening the steam-exposed tray prior to instrument removal. Process indicators, such as labels and autoclave tapes, indicate that a wrapped instrument tray or other wrapped article or articles has been exposed to steam.
Internal chemical indicators and process indicators, such as autoclave tapes, should indicate (by color change) the presence of steam under various operating conditions but should remain their original color, or near their original color, under other conditions in which inadequate amounts of steam for sterilization of medical devices has contacted them. For example, an autoclave tape should show a significant color change when subjected to steam in a hospital autoclave at 132-134° C. for around 3 or 4 minutes and at 121° C. for around 20 minutes. Additionally, when tested using a steam resistometer, and according to ISO Standard 11140, an autoclave tape should show a significant color change on contact with steam at 134° C. for 2 minutes, and at 121° C. for 10 minutes, but should not show a significant color change on exposure to steam at 134° C. for 30 seconds nor at 121° C. for 3 minutes. Finally, an autoclave tape should not show a significant color change on exposure to dry heat at 140° C. for 30 minutes.
Steam sterilization indicator compositions for both internal indicators and process indicators that have been used include a polyvalent metal compound, such as lead carbonate, and sulfur. Such indicators turn to brown or black when their color is fully developed by a steam sterilization condition. Because of environmental concerns, lead compounds have been and continue to be replaced by other polyvalent metal compounds. For example, bismuth has been proposed to replace lead in certain steam sterilization indicator compositions as described in U.S. Pat. No. 5,916,816 (Read) wherein, for example, bismuth subcarbonate was used.
Accordingly, there is a continuing need for lead-free chemical indicators that can indicate that a steam sterilization process condition has been met.